If this is your first visit, be sure to
check out the FAQ by clicking the
link above. You may have to register
before you can post: click the register link above to proceed. To start viewing messages,
select the forum that you want to visit from the selection below.

More ways than one to skin a cat

10-22-2012, 04:30 PM

Some might recall that I posted some pics of a diesel engine heat exchanger repair that I did a couple of years ago. The exchanger consists of a cast aluminium tank with two tube stacks, a large one for water/coolant and a smaller one for oil. They rely on a single O-ring on each stack end to keep raw (salt) water, which passes through the tubes, out of the engine coolant and oil and, just as important, away from the bronze/aluminium junction at the ends of the stacks. Inevitably after 45 years service this seal has leaked, more than once in the life of the engine.

This is the worse end of the tank:-

The previous one I had bored out the ends in the mill, and made 316 inserts for the ends out of solid bar (very wasteful). That was for a 6-cylinder engine, the latest job was for an 8-cylinder version of the same engine and so bigger, and too tall to fit under the head of my mill. If I removed all the fittings from the tank it would just have clearance to swing in the (19") lathe, but how to hold it? I'd thought about making a wooden mandrel - the bore for the tube stack is 5.75" - but didn't have anything suitable lying around. Supporting it was easy enough, gripping inside with a 3-jaw on the lathe spindle and with a revolving 3-jaw in the tailstock, but the ends needed to be bored out. In fact I found that I could tighten the little 3-jaw inside the tank bore, fairly tight, and simply screw the chuck further in with the tailstock ram without difficulty. Luckily the tailstock on my big lathe has a good length of extension available, so it would go far enough in to allow boring tools alongside the ram.
One of my concerns doing it this way was that there was nothing holding the big unbalanced lump positively against the chuck jaws on the main spindle, where the engagement of the jaws was fairly limited, but there was never the slightest sign of it trying to come away.

I started gingerly, but then by pushing the tailstock chuck further in was able to use a decent sized boring tool:-

For the previous job, I'd enquired about getting some sort of 316 tube to do the job but was told there was nothing available & so had to bore out solid bar.
This time, the stockholder persuaded me that some 6" schedule 80 pipe would do the job. The dimensions he quoted looked just about OK, so I ordered a piece. In fact, the ID was a bit bigger than he had quoted, might just about have been useable except that it was way out of round and nowhere near concentric with the OD.
There was no time to mess about arguing with the supplier and re-ordering, so I built up the critical area on the inside with some TIG welding (it just has to hold an O-ring under compression) and machined it out. My TIG skills are not very advanced, but good enough to get the job done.

Here is one of the inserts, and then fitted and with the water stack in place:-

There was some corrosion on one end of the seating for the oil stack, it wasn't too bad so I repaired that with some Devcon Aluminium epoxy putty.

Very cool job... Bet there was some SERIOUS pucker factor going on spining that on the lathe... I saw the blur in the picture and was like... No... No.. hes not spining that.. its line boring.. hes just... No... He can't be.... Ahh **** he is.

Don't quiet get how the heat exchanger works however, Is that grill in the last picture filled with little tubes that go to the other seal? And then engine water/oil fed around all the little tubes?

Comment

This type of job can sometimes be done more easily on a horizontal mill. Workpiece clamped to the table, boring bar mounted in place of the horizontal arbor. A poor man's HBM.

One other option was to do it horizontally on my mill, which is a combination turret/horizontal machine, with a boring & facing head. There would have been lots of the casting overhanging the front of the table (no big deal, it's not terribly heavy) and would have meant working from the back of the machine, you can reach the controls OR see what you're doing, not both at the same time. Perfectly feasible, it was the first option I considered when I realised it wouldn't fit under the head, but decided that if I could set it up OK in the lathe it should be quicker & easier.
Another option might have been to swing the turret head round to the horizontal, that way the casting would have lain along the table and visibility would have been fine. I didn't investigate in detail how feasible it would have been, probably would have needed to sit the work on a box cube, but one of the things which keeps machining work interesting is that often there are several ways to approach a job (hence my title for the thread), and you the person doing the job get to decide the best way in your own circumstances.

Tim

Comment

Comment

A bit of video of the engine here under test after finishing the job. Heat exchanger is mounted front top of the engine.
It's 240 shp @ 1000 rpm, running here under load at 600 rpm. 31 litres capacity. Built about 1964, replaced steam plant. Later versions of the same engine, with the same bore and stroke but with turbocharging and strengthened crankcase, were rated up to 500 shp and more.
The jangling noise is the mechanical clutch for the cargo pump, which is driven from the front of the crankshaft (and can take around 60 hp).